In general, a lithium ion battery obtains electric energy by giving and receiving lithium ions between a positive active material and a negative active material. These lithium ions move from the positive active material to the negative active material through an electrolyte solution during charging and inversely move from the negative active material to the positive active material through the electrolyte solution during discharging. The lithium ions having moved to the negative active material receive electrons via a negative electrode member, so that lithium is doped into the negative active material.
In the case where a lithium ion battery is manufactured by using a negative active material that has not previously been doped with lithium, lithium does not initially exist in the negative active material and thus the battery cannot sufficiently discharge and act as a battery. Therefore, the battery has to be subjected to initial charge to dope lithium into the negative active material. This initial charge is performed by applying a predetermined voltage between a positive active material and a negative active material. At that time, however, a sold electrolyte interface (SEI) tending to disturb giving and receiving of lithium ions may be formed on an electrode surface and part of the lithium ions may be doped accidentally in a region of the negative active material that cannot contribute to charge and discharge reactions. SEI represents a film or layer that is generated by side reaction of battery reaction and a formation state thereof differs according to a material of the negative active material and a composition of the electrolyte solution. Those phenomena are irreversible reactions. Accordingly, during discharge following the initial charge, a discharge capacity tends to become smaller than an initial charge capacity. To reduce a capacity difference (irreversible capacity) caused by the irreversible capacity between the charge capacity during the initial charge and the discharge capacity during subsequent discharge, the battery has only to ensure an excessive amount of lithium or lithium ions by assuming that partial lithium or lithium ions of the lithium doped into the negative active material cannot be released during discharge.
A conceivable technique thereof is to make a positive electrode member carry or support an excessive amount of positive active material. However, this results in an increase in absolute mass of the positive active material provided in the positive electrode member and an increase in thickness of a positive active material layer from which lithium is released. This may lead to a problem that resistance in the positive electrode member increases (internal resistance of a battery increases).
Patent Literature 1 proposes that metal lithium is placed inside a case body of a lithium ion battery so that the metal lithium is electrically connected to a negative active material through a conduction member. This metal lithium supplies an excessive amount of lithium ions to the negative active material, separately from lithium ions existing in the positive active material.
According this technique, when an electrolyte solution is poured into the battery case in which the metal lithium is placed, the metal lithium is dissolved in the electrolyte solution in the form of lithium ions by a potential difference from the negative active material, and then the lithium ions are doped into the negative active material. Thus, the capacity difference (irreversible capacity) between the initial charge capacity during initial charge performed subsequently and a subsequent discharge capacity will practically be eliminated. It is possible to appropriately distribute adjustment weight of the positive active material and the negative active material, thereby enhancing energy density (see Patent Literature 1).
Patent Literature 1: JP8 (1996)-102333A